Domain Name System (DNS)

1

CSC465 – Computer Networks

Spring 2004

Dr. J. Harrison

These slides are based on the text “TCP/IP Protocol Suite (2nd_Edition)”

Chapter 18

Domain Name

System

(DNS)

CONTENTS

CONTENTS

• NAME SPACE

• DOMAIN NAME SPACE

• DISTRIBUTION OF NAME SPACE • DNS IN THE INTERNET • RESOLUTION • DNS MESSAGES • TYPES OF RECORDS • COMPRESSION • EXAMPLES • DDNS • ENCAPSULATION

Domain Name Space

• Hierarchical name space

• Names are designed with inverted-tree structure (root at top)

• Max 128 levels

• Each level of tree defines a hierarchical level in the name space

• Each node in tree has a label with max 63 chars • Root label is null string

• To ensure uniqueness, DNS requires that children of node have different labels

Domain Name Space

• Each node in the tree has a domain name • Full domain name is a sequence of labels

separated by dots

• Domain names read from node to root • Last label is label of root (null)

• All full domain names end with a dot (“.”)

Domain is a subtree of the domain name space. The name of a domain is the domain name of the node at the top of the domain.

Domain names and labels

A node can be in multiple domains

pa.ca.us. is in both ca.us. domain and us. domains

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DISTRIBUTION

OF

NAME SPACE

DNS in the Internet

Generic domains Country domains

Hierarchy of name servers

Centralization of DNS?

• single point of failure • traffic volume

• distant centralized database • maintenance

Delegation

• key goals of the design of the Domain Name System was to decentralize administration (achieved via delegation) • an organization administering a domain can divide it into

subdomains

• Each subdomain can be delegated to another organization • Each organization becomes responsible for maintaining

all the data in that subdomain

– can freely change the data and even divide its subdomain up into more subdomains and delegate those

– The parent domain contains only pointers to sources of the subdomain's data

Name Server

• programs that store information about the domain name space are called name servers

• Name servers generally have complete

information about some part of the domain name space, called a zone

• A name servers can load DNS data from a file or from another name server

– it then has authority for that zone

• Name servers can be authoritative for multiple zones

Name Server

• no server has all name-to-IP address mappings

local name servers:

– each ISP, company has local (default) name server

– host DNS query first goes to local name server authoritative name server:

– for a host: stores that host’s IP address, name – can perform name/address translation for that host’s

name

Zones vs. Domains

• All top-level domains, and many domains at the second level and lower, like unlv.edu and sun.com, are broken into smaller, more manageable units by delegation (called zones)

• otherwise, the domain manager would have to manage the subdomains themselves

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Zones and domains _{Example: All Canadian states have responsibility for their}

own subdomains; complete delegation of subdomains

Example: Two Canadian states managed within the ca

zone; other 3 have their subdomains delegated to them

Name Server Types

• DNS specs define two types of name servers:

primary and secondary

• A primary name server for a zone reads the data for the zone from a file on its host

• A secondary name server for a zone gets the zone data from another name server that is authoritative for the zone (called its master server)

– a secondary can load zone data from another

secondary, which would be termed its master

• When a secondary starts up, it may pulls the zone data over from its master server (zone transfer)

Name Server Types

• Both the primary master and slave name servers for a zone are authoritative for that zone • Slave name servers are important because it's a

good idea to set up more than one name server for any given zone

• a name server can be authoritative for >1 zone • a name server can be a primary master for one

zone and a slave for another

Resolvers

• clients that access name servers

• Programs running on a host that need information from the domain name space use the resolver • The resolver handles:

– Querying a name server

– Interpreting responses (which may be resource

records or an error)

– Returning the information to the programs that requested it

Resolution

• Name servers are adept at retrieving data from the domain name space

• they provide data about zones for which they're authoritative

• they can also search through the domain name space to find data for which they're not authoritative

• process is called name resolution

Root Name Servers

• know where there are authoritative name servers for each of the top-level domains

– most are authoritative for the generic top-level domains

• top-level name servers can provide the list of name servers that are authoritative for the second-level domain

• Each name server queried gives the querier info about how to get "closer" to the answer it's seeking, or it provides the answer itself.

Root Name Servers

• if no caching, resolution would have to start at the root name servers

• root name servers crucial to the operation of DNS • if all the Internet root name servers were

unreachable for an extended period, all resolution on the Internet would fail

• ∴∃ 13 for redundancy

“The”13 Root Name Servers

e NASA Mt View, CA f Internet Software C. Palo Alto, CA

i NORDUnet Stockholm k RIPE London

m WIDE Tokyo a NSI Herndon, VA

c PSInet Herndon, VA d U Maryland College Park, MD g DISA Vienna, VA h ARL Aberdeen, MD j NSI (TBD) Herndon, VA

root name server

local name server dns.eurecom.fr 1 2 3 4 5 6 7 8 Root name server:

❒may not know

authoritative name server

❒may know

intermediate name server:who to contact to find authoritative

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DNS: Iterated Queries

recursive query:

• puts burden of name resolution on contacted name server • heavy load? iterated query: • contacted server replies with name of server to contact • “I don’t know this

name, but ask this server”

requesting host surf.eurecom.fr

gaia.cs.umass.edu root name server

local name server dns.eurecom.fr 1 2 3 4 5 6

authoritative name server dns.cs.umass.edu intermediate name server

dns.umass.edu 7 8 iterated query Recursive resolution Iterative resolution

DNS: caching and updating records

• once (any) name server learns mapping, it

cachesmapping

– cache entries timeout (disappear) after some time (TTL field)

• update/notify mechanisms under design by IETF – RFC 2136

– http://www.ietf.org/html.charters/dnsind-charter.html

DNS Query and Response Messages _{Header format}

16-bit field Identification used by the client to match the response with the query

Client uses a different ID# each time its sends a query Server duplicates this number in the corresponding response

QR: Query/Response

OpCode: 0 standard, 1 inverse, 2 server status AA: Authoritative (was responding server?) TC: Truncated (too large response for UDP) RD: Recursion Desired (both query & response) RA: Recursion Available (only response) rCode: Status of the error

Flags fields

TYPES

OF

RECORDS

Question record format

admin.atc.fhda.edu.

(1 = IN)

Some Query Types

• Type=A;host name to host IP

– Ex: (relay1.bar.foo.com, 145.37.93.126, A) • Type=NS;domain to an authoritative name

server for domain

– Ex: (foo.com, dns.foo.com, NS)

• Type=CNAME;provides canonical hostname for alias hostname

– Ex: (foo.com, relay1.bar.foo.com,CNAME) • Type=MX;canonical name of a mailserver

– Ex: (foo.com, mail.bar.foo.com, MX)

Resource record format

(ptr offset to domain name in question record)

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Example 1

Example 1

A resolver sends a query message to a local

server to find the IP address for the host

“chal.fhda.edu

.”. We discuss the query and

response messages separately.

Example of a query message

(A) _(Internet) ♦ query recs Response Message 13 (Address) “Internet” ⇒ TTL (secs) length⇒ bytes 153.18.8.105 ♦ ♦ ♦

query recs answer Example 2Example 2

An FTP server has received a packet from an

FTP client with IP address 153.2.7.9. The

FTP server wants to verify that the FTP

client is an authorized client.

The list of authorized clients are specified as

domain names. The FTP server asks the

resolver (DNS client) to send an inverse

query to a DNS server to obtain domain

name.

Example of Inverse Query Message OpCode 1 for “Inverse” ♦ ♦ ♦ query recs (Internet) Example of inverse response message “in-addr.arpa” is added to domain space 153.2.7.9 ⇒ “9.7.2.153.in-addr.arpa” 12 ⇒ query type is PTR

DNS can use the services of

DNS can use the services of

UDP or TCP

UDP or TCP

using the well

using the well

-

-

known port 53.

known port 53.